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I have an existing deck that I am refurbishing. The deck is only 18" off the ground. The existing joists are in good shape. I would like to use shorter deck boards with abutted ends due to the length of the deck boards. The existing joists are long as well with supports running underneath them.

Instead of adding a full length sister joist I am wondering if two half sister joists with blocking in between is a viable alternative? The main reasons are that the longer sister joist would be a custom order at the lumber yard and more difficult to transport.

Below is a simplified drawing. I'm actually considering adding two pair of the half sister joists and blocking so the deck boards can be in a staggered pattern. That way there wouldn't be a seam down the middle.

enter image description here

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  • I don't understand why you think you need the sister joists down the center. Is that so you can fasten one deck board to the main joist, and the abutting deck board to the sister joist? – SteveSh Jan 25 '20 at 19:29
  • Yes that's correct. It's to fasten one to the main joist and the other to the sister joist. – user1934580 Jan 25 '20 at 19:38
  • Use stainless steel screws. – JACK Jan 25 '20 at 20:09
  • I’m using Trex hidden fasteners to attach the deck boards to the joists. – user1934580 Jan 25 '20 at 20:24
  • using shorter boards seems like a hassle, if it's about transport many hardware suppliers will deliver for a nominal fee. but yeah if it's recycles lumber then you don't get so many size options. – Jasen Jan 25 '20 at 20:31
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As the sister joist is not to reinforce the structure of the deck, but only to support the ends of the floor planks then making if from multiple short pieces will not be a problem.

it doesn't even need to be the full size of the other joists, you can use 2x4 or whatever. is convenient.

It's also legal to rest two plank ends on a single joist (each end about half way across) but if your deck is too long for your planks to be used that way you can't do that.

I'd consider staggering the boards so that all the joins don't happen in a straight line.

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  • I should have mentioned that I’m using Trex boards which require abutted boards to be attached to separate joists. Yes I believe I could have used the same joist if I were using pretreated deck boards. I will likely stagger the deck boards as you suggest. – user1934580 Jan 25 '20 at 20:36

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